Python中从类调用列表显示对象地址而非内容的问题解决

问题描述 #

从Owner类调用self.pets列表时，输出的是Pet对象的内存地址信息，而非预期的宠物名称：

当前输出：

David Friend's pets are: [<__main__.Pet object at 0x7f547c620128>, <__main__.Pet object at 0x7f547c620198>]
Audrey Hepburn's pets are: [<__main__.Pet object at 0x7f547c620208>]

预期输出：

David Friend's pets are: Boggle Joyner, Artemis Joyner
Audrey Hepburn's pets are: Pippin Hepburn

原代码如下：

class Name:
    def __init__(self, first, last):
        self.first = first
        self.last = last

class Pet:
    def __init__(self, name, owner):
        self.name = name
        self.owner = owner
        
class Owner:
    def __init__(self, name):
        self.name = name
        self.pets = []

def get_pets_string(an_owner):
    return str(an_owner.name.first) + " " + str(an_owner.name.last) + "'s pets are: " + str(an_owner.pets)


owner_1 = Owner(Name("David", "Friend"))
owner_2 = Owner(Name("Audrey", "Hepburn"))

pet_1 = Pet(Name("Boggle", "Joyner"), owner_1)
pet_2 = Pet(Name("Artemis", "Joyner"), owner_1)
pet_3 = Pet(Name("Pippin", "Hepburn"), owner_2)

owner_1.pets.append(pet_1)
owner_1.pets.append(pet_2)
owner_2.pets.append(pet_3)

print(owner_2.name.first)
print(get_pets_string(owner_1))
print(get_pets_string(owner_2))

原因分析 #

Python默认打印对象时，会输出对象的类名和内存地址，这是因为没有为Pet和Name类定义自定义的字符串表示方法。

解决方法 #

方法一：为类添加__str__方法 #

__str__方法用于定义对象被转换为字符串时的显示内容，添加后Python会自动使用这个方法的返回值来打印对象。

1. 给Name类添加__str__方法，返回完整姓名：

class Name:
    def __init__(self, first, last):
        self.first = first
        self.last = last
    
    def __str__(self):
        return f"{self.first} {self.last}"

2. 给Pet类添加__str__方法，返回宠物的姓名：

class Pet:
    def __init__(self, name, owner):
        self.name = name
        self.owner = owner
    
    def __str__(self):
        return str(self.name)

3. 修改get_pets_string函数，将宠物列表转换为逗号分隔的字符串：

def get_pets_string(an_owner):
    pets_str = ", ".join(str(pet) for pet in an_owner.pets)
    return f"{an_owner.name.first} {an_owner.name.last}'s pets are: {pets_str}"

方法二：直接在函数中处理宠物名称（无需修改类） #

如果不想修改原有类的结构，可以直接在get_pets_string函数里遍历宠物列表，提取每个宠物的姓名并拼接：

def get_pets_string(an_owner):
    pets_str = ", ".join(f"{pet.name.first} {pet.name.last}" for pet in an_owner.pets)
    return f"{an_owner.name.first} {an_owner.name.last}'s pets are: {pets_str}"

修改后的完整代码（方法一版本） #

class Name:
    def __init__(self, first, last):
        self.first = first
        self.last = last
    
    def __str__(self):
        return f"{self.first} {self.last}"

class Pet:
    def __init__(self, name, owner):
        self.name = name
        self.owner = owner
    
    def __str__(self):
        return str(self.name)
        
class Owner:
    def __init__(self, name):
        self.name = name
        self.pets = []

def get_pets_string(an_owner):
    pets_str = ", ".join(str(pet) for pet in an_owner.pets)
    return f"{an_owner.name.first} {an_owner.name.last}'s pets are: {pets_str}"


owner_1 = Owner(Name("David", "Friend"))
owner_2 = Owner(Name("Audrey", "Hepburn"))

pet_1 = Pet(Name("Boggle", "Joyner"), owner_1)
pet_2 = Pet(Name("Artemis", "Joyner"), owner_1)
pet_3 = Pet(Name("Pippin", "Hepburn"), owner_2)

owner_1.pets.append(pet_1)
owner_1.pets.append(pet_2)
owner_2.pets.append(pet_3)

print(owner_2.name.first)
print(get_pets_string(owner_1))
print(get_pets_string(owner_2))

运行上述代码后，就能得到预期的输出结果。

内容的提问来源于stack exchange，提问作者Frederic